1. Technical Field
The present invention relates to phototransistors in general, and in particular to a depletion-mode field-effect transistor-based phototransistor.
2. Description of Related Art
While the scaling down of dimensions of electronic devices has provided dramatic improvements in their performance, on-chip/off-chip interconnects have become a major performance bottleneck because the scaling also reduces cross-sectional dimensions. As a result, conventional schemes for metal interconnects become problematic in terms of latency, bandwidth and power.
The usage of optical interconnects can avoid the resistive loss and capacitive physics of metal interconnects. Optical interconnects can also help to meet the latency and bandwidth requirements. However, the energy per bit of the optical interconnects is still too high compared to that of their metal counterparts except for long wires. While some modulator schemes promise to satisfy power requirements for on-chip interconnects, lasers and receiver circuits still have technological problems. These problems are inherent in photodiodes that are commonly used as photodetectors in optical receiver circuits. Because of the diffraction limit of light, a photodiode is not scalable, and therefore a device with sub-wavelength dimensions suffers from reduced responsivity. In addition, photodiodes have a much larger footprints than many electronic devices, which may lead to large output capacitances, large dark currents, and high power consumption in receiver circuits.
In addition, typical photodiodes have a theoretical limit on the external quantum efficiency (EQE) of 1, or a responsivity of ˜0.7 A/W at 850 nm wavelength. This limitation and the large dark current require a high optical input signal in order to maintain a sufficient signal-to-noise ratio. Thus, optical emitters can consume a large amount of power. Even though previous studies on phototransistors have shown that they have a high responsivity, those devices suffered from increased dark current and a longer response time.
Rather than collecting generated electron-hole pairs directly, it is possible to use the carriers to change the band bending in the device, and thereby modulate the potential and thus the output current. Theoretically, this method has the potential to overcome theoretical limitation on EQE, but in practice, it has been impossible to demonstrate that the above-mentioned method has a better performance than that of photodiodes. Furthermore, since these photodiodes are not scalable, large output capacitances are still problematic.
Consequently, it would be desirable to provide an improved phototransistor to be used in conjunction with optical interconnects.